This invention relates to communication between Local Area Networks (LANs) and, particularly but not exclusively, between Token Rings (TRs) which are a particular type of LAN.
A LAN is a digital communications network interconnecting several local computer workstations. Different types of LANs have been proposed including a serial loop configuration in which several adapters are arranged serially about the periphery of the loop with respective data sources connected to the adapters. In this configuration data flows from a data source through its associated source adapter and serially through one or more intervening adapters until it reaches the target adapter where it is extracted and forwarded to the target data source. Such a serial loop LAN is the Token Ring LAN developed by IBM.
As described in U.S. Pat. No. 4,621,362 of Nov. 4, 1986 and IBM Token Ring Network: Architecture Reference, First Edition February 1986, Publication #6165877, both of which are incorporated herein by reference such Token Ring LANs can be connected with one another using full bridges to form a multiple Token Ring LAN. However, interconnection by full bridges requires that the Token Rings be located physically proximate one another.
United Kingdom published patent Application No. 2,170,079, published July 23, 1986, incorporated herein by reference, describes in general terms a procedure for interconnecting remote LANs across broadcast simplex channels which uses half-bridges which encapsulate the lowest layer frames of the LAN in data-link-layer frames. However, the half-bridges described in that published application do not implement what is termed SOURCE ROUTING, i.e, each bridge through which a frame is routed identifies itself and the associated ring. Moreover, the half-bridges assume all routing between half-bridges is done by the half-bridges themselves and cannot operate in a hubbed configuration in which the hub is some generalized switching mechanism possibly taking the form of a Wide Area Network.